Last month, the prime minister’s spokesman said if Mr. Johnson was unwell and unable to work, Mr. Raab, as the first secretary of state, would stand-in.

Mr. Jenrick said: “We hope that as a result of these tests [the prime minister] will be able to come back to Downing Street as soon as possible.

“He’s been working extremely hard leading the government and being constantly updated. That’s going to continue.”

“I’m sure this is very frustrating for him, for somebody like Boris who wants to hand [on] running the government from the front, but nonetheless he’s still very much in charge of the government,” he added.

US President Donald Trump is among those who has sent his wishes to Johnson.

“All Americans are praying for him. He’s a great friend of mine, a great gentleman and a great leader,” Trump said, adding that he was sure the prime minister would be fine because he is “a strong person”.

And Labour leader Keir Starmer said he hoped the prime minister had a “speedy recovery”.

Health Minister Nadine Dorries, who herself tested positive for coronavirus last month, said many of those with the virus would be “felled” by fatigue and high temperature and use isolation to sleep and recover.

“Boris has risked his health and worked every day on our behalf to lead the battle against this vile virus,” she said in a tweet.

Although Downing Street insists the PM is still in charge if the medics insist he needs to rest and recuperate then he may well have to take a step back for a period of time.

In the UK we no longer have a deputy prime minister – the last one was Nick Clegg under David Cameron.

Technically, Dominic Raab – as the first secretary of state – would be expected to step up.

His position as foreign secretary, however, does not put him at the center of the fight against coronavirus.

It would seem likely therefore that the two figures who would be expected to take a key role would be Cabinet Office Minister Michael Gove and Health Secretary Matt Hancock.

Both men have been frequent representatives of the government at the daily news conferences.

Fortunately, however, many of the key decisions – namely how long the lockdown should continue and what the exit strategy should be – still appear some way off.

At the moment the focus is on “flattening the curve” and reducing the level of infections and admissions to the hospital.

The day-to-day decisions are therefore likely to depend much more on the advice of the scientists and officials.

The really big decisions are some way off – when it is hoped the prime minister will have recovered.

In other developments:

– Scotland’s chief medical officer has resigned after making two trips to her second home – despite government guidance urging people to avoid unnecessary travel

– The National Domestic Abuse helpline has seen a 25% increase in calls and online requests for help since the lockdown, the charity Refuge says

– High street pharmacists are “needlessly being put at risk” due to a lack of personal protective equipment (PPE), the Royal Pharmaceutical Society says

– Train drivers’ union Aslef is also calling for London Underground drivers to be provided with masks and gloves to protect them from contracting COVID-19

– Young workers and the worst paid are the most likely to be affected by the closure of businesses because of coronavirus, according to research by the Institute for Fiscal Studies

On Sunday the Department of Health said 621 more people had died in hospital in the UK after testing positive for coronavirus, taking the total death toll to 4,934.

As of 09:00 BST on Sunday, 47,806 people had tested positive for coronavirus, the department said.